The present invention relates to ink compositions useful for thermal transfer printing processes. More specifically, the present invention is directed to thermal transfer elements with inks containing mesomorphic materials, primarily those molecules and polymers that exhibit liquid crystalline behavior.
Thermal printing is a nonimpact printing process that enables formation of high resolution images. These printing processes are simple, offer low noise levels, and are very reliable over extended usages. Thermal printing processes may be classified into three categories. Direct thermal printing entails the imagewise heating of special papers coated with heat sensitive dyes, such that an image forms in the heated areas. Another method of thermal printing is known as the dye transfer or dye sublimation technique, and operates by heating a transfer element coated with a sublimable dye, which transfer element is not in contact with the receiving substrate. When the transfer element is imagewise heated, the dye sublimates and migrates to the receiver sheet, which possesses a polymeric coating into which the dye diffuses, forming an image. A third method of thermal printing is known as thermal transfer printing. The thermal transfer printing process entails imagewise heating of a transfer element containing ink, which transfer element is in intimate contact with the heater on one side and the receiving substrate on the other side. Imagewise heating of the transfer element affects the ink in such a way as to cause it to transfer from the transfer element to the receiving substrate, thereby resulting in image formation. Thermal transfer printing methods generally employ uncoated plain papers, which enables prints with acceptable appearance and excellent archival properties. In addition, the thermal transfer printing method can be employed for color printing applications by using transfer elements of the desired color or colors.
Thermal transfer printing processes generally employ a thermal printhead, a transfer element, and a receiver sheet. The side of the transfer element containing the ink is placed in contact with the receiver sheet, and heat originating from the printhead is then applied to the transfer element. Heat conducted through the transfer element increases the temperature of the ink, which may cause it to melt, soften, decrease in viscosity, or otherwise undergo a transition that enables the ink to transfer to the receiver sheet. After the receiver sheet and transfer element are separated, an image remains on the receiver sheet. An alternative method of heating the transfer element, known as resistive heating, employs an array of electrodes instead of thermal printhead to generate a current between the electrodes and a grounded conductive layer in the transfer element. This method is described in the IBM Journal of Research & Development, Vol. 29, No. 5, 1985, the disclosure of which is totally incorporated herein by reference. Additional information concerning thermal transfer printing processes is disclosed in Thermal Transfer Printing: Technology, Products, Prospects, published by Datek Information Services, P.O. Box 68, Newtonville, MA, the disclosure of which is totally incorporated herein by reference.
The thermal transfer printing process has been disclosed in, for example, U.S. Pat. No. 3,441,940 and U.S. Pat. No. 3,745,586, the disclosures of each of which are totally incorporated herein by reference. In addition, augmented thermal transfer printing processes are known. For example, U.S. Pat. No. 3,989,131 discloses a pressure assisted thermal transfer printing process employing an electrothermic printing unit for writing dot matrix characters on a printing line of recording medium by means of an electrothermal printing head which is continually movable along the printing line. Pressure is interposed between the head and the recording medium, pressure means being provided for pressing the printing elements against the transfer element and the receiver sheet. In addition, U.S. Pat. No. 4,541,042 discloses a transfer recording process assisted by a solvent, wherein a receiving medium such as paper and an ink transfer sheet are placed in contact between a platen and a thermal head, and a liquid, volatile solvent is applied to the paper. The solvent enables high speed thermodissolving transfer of the ink to the paper by heating selected areas to form an image.
Further, U.S. Pat. No. 4,525,722, discloses a thermal transfer printing process assisted by chemical heat amplification, wherein some of the heat necessary for melting and transferring the ink from a solid fusible layer in a ribbon to a receiving medium is provided by an exothermic reaction involving an exothermic material contained in a layer in the ink ribbon. Also, U.S. Pat. No. 4,549,824 discloses a thermal transfer printing process aided by an exothermic reaction, wherein an aromatic azido compound is added to the ink, said azido compound being one that exotherms at the conditions of thermal ink transfer. In addition, U.S. Pat. No. 4,550,324 discloses an ink transfer thermal printer utilizing a thermosensitive ink that is solid at normal temperatures, with selected portions of the ink being liquefied by heating and transferred onto recording paper. The printer can be of either contact or non-contact (ink jet) configuration, and eliminates the need to utilize disposable materials such as ink ribbons.
U.S. Pat. No. 4,567,489 discloses a thermal printhead for a thermographic printer having an electrically insulating substrate on which resistors are placed that form impression points and current supply and current discharge leads bonded to the resistors. The printhead includes a structure for forming a magnetic field that acts on the resistors in the immediate proximity of the resistors and along the resistor print line. The magnetic field is directed such that when the current flows through the resistors, the current paths are deflected upward into the upper part of the resistor on its outer surface. The single resistor impression points thus reach their highest temperature at the printing surface where they must deliver heat to the recording medium, which results in the heat needed for heating the resistor being supplied more quickly to the recording medium, thereby reducing the cooling time of the single resistor impression point so that a higher printing velocity can be attained with the thermal printhead.
Additionally, U.S. Pat. No. 4,510,511 discloses a picture recording method and apparatus using an ink containing an evaporable coloring matter, which enables printing on a medium without an ink ribbon. The special ink is supplied to an ink transporting means and then cooled below the melting point of the ink bonding agent. A discharge energy is applied, controlled according to the picture to be formed, which causes the coloring matter to fly to the recording medium opposite the transporting means. Essentially, the process entails fluidizing a marking material by heat, picking up the liquid marking material on a gravure type roll, and selectively transferring it to the receiving sheet by means of a high voltage field.
U.S. Pat. No. 4,046,073, the disclosure of which is totally incorporated herein by reference, discloses a printing or copying system in which ink is transferred from an ink-bearing medium, which can be a porous medium filled with ink in the pores, to a printing medium through the use of ultrasonics. The ink bearing medium is placed in contact with the paper and ultrasonic energy is applied to the medium, causing the ink to decrease in viscosity due to ultrasonic vibrations and conversion of ultrasonic energy into heat. The ink is then transferred to a printing medium. In addition, European Patent Application 0,254,420 discloses a recording method wherein a recording member generally in the form of a cylinder and having on its outer surface one or more porous layers impregnated with ink is heated on the outer surface in image configuration. Ink contained in the cylinder is thus brought to the surface of the imaging member and transferred to a recording sheet in imagewise fashion.
Further, U.S. Pat. No. 4,803,119, the disclosure of which is totally incorporated herein by reference, discloses ink coating compositions for impact typewriter ribbons, which ink coatings comprise a sponge material having dispersed therein an ink comprising pigment particles and a dimer acid. Further, U.S. Pat. No. 3,348,651, the disclosure of which is totally incorporated herein by reference, discloses pressure sensitive ink transfer ribbons, tapes, and sheets having a microporous inking composition for use in typewriters, high speed printers, and optical scanning devices. The pressure sensitive ink transfer medium comprises a shock-absorbent base layer of an elastomeric polymer film having a high degree of resiliency in a direction normal to the plane of the film, an intermediate layer of a thin, non-elastic polymer film bonded to the base layer, and an inking layer bonded to the intermediate layer over substantially its entire working surface and comprising a substantially continuous film of a microporous inking composition. The microporous inking composition consists essentially of a uniformly blended mixture of an elastomeric polymeric binder, an inking compound comprising a non-aqueous, non-volatile ink carrier which is substantially insoluble in the elastomeric polymeric binder and which contains a high concentration of an ink pigment, and a finely ground microporous inorganic filler. Other patents, such as U.S. Pat. Nos. 3,287,153, 3,392,042, 3,484,508, 3,930,099, 4,321,286, 4,544,292, and 4,624,881, also disclose pressure sensitive porous marking ribbons filled with an exudable marking material. In addition, U.S. Pat. Nos. 3,351,948, 3,847,265, 4,251,276, 4,414,555, 4,415,903, 4,603,986, 4,608,577, 4,762,734, 3,480,962, 4,128,345, 4,205,320, and U.S. Pat. No. 4,315,267 are of background interest.
One problem encountered in thermal transfer printing is the necessity of imagewise heating the ink, which typically comprises a crystalline wax, through the melting phase. The latent heat of melting and the time necessary for heat transfer for this melting phase change places both power and speed constraints on the thermal transfer printing process, which constraints influence printer performance and manufacturing cost. Replacing the ink containing a crystalline wax, which requires crystalline melting, with an ink of the present invention, which can be transferred to a substrate by means of a mesomorphic transition, will improve upon these shortcomings, since mesomorphic transitions typically require far less heat to image a pixel or pixels selectively.
For example, the heat of fusion of a typical paraffin or long chain aliphatic hydrocarbon wax is approximately 39 kilocalories per gram, while the heat of mesomorphic transition of a typical liquid crystalline material such as p,p'-azoxyanisole is approximately 0.7 kilocalorie per gram. Typically, thermal transfer printing processes entail heating the transfer element from room temperature of about 20.degree. C. to a temperature of from about 40.degree. to about 95.degree. C. Thus, heating one gram of a paraffin wax with a heat capacity of about 0.5 kilocalorie per degree Celsius per gram from 20.degree. C. to 60.degree. C. requires about 59 kilocalories per gram, melting the wax at that temperature requires about 39 kilocalories per gram, and heating the melted wax from 60.degree. C. to 70.degree. C. requires an additional 5 kilocalories per gram, for a total energy requirement of about 64 kilocalories per gram. In comparison, heating one gram of a liquid crystalline material such as p,p'-azoxyanisole with a heat capacity of about 0.5 kilocalorie per degree Celsius per gram from 20.degree. C. to 60.degree. C. requires about 20 kilocalories per gram, inducing a mesomorphic transition in the liquid crystalline material at that temperature requires about 0.7 kilocalorie per gram, and heating the material from 60.degree. C. to 70.degree. C. requires an additional 5 kilocalories per gram, for a total energy requirement of about 25.7 kilocalories.
When a liquid crystalline material is heated and undergoes a mesomorphic transition, its viscosity can drop by as much as five orders of magnitude (e.g. from about 100,000 centipoise to about 10 centipoise) during the process of a mesomorphic change occurring over a 10.degree. C. temperature range. Thus, imagewise heating of a transfer element containing materials that undergo mesomorphic transitions will result in a change in the material's rheological properties sufficient to effect imagewise transfer of the ink to a receiver sheet.
Ink compositions containing liquid crystalline materials are suitable for use in conjunction with both single use ink transfer elements and multi-use ink transfer elements. Multi-use thermal transfer elements such as those disclosed in copending application U.S. Ser. No. 454,800; the disclosure of which is totally incorporated herein by reference, generally require more energy to heat than do single use transfer elements since they possess a layer of sponge material within which the ink is contained, in addition to a supporting substrate, and also contain more ink than do single use elements in order to enable multiple uses. Accordingly, providing a thermal transfer ink composition that requires less heat to enable image formation than that required by conventional thermal transfer inks provides more energy efficient imaging. Ink compositions with lowered heating requirements also enable the provision of multi-use elements with thicker sponge layers, since although heat dissipates as it passes through the thick sponge layer, enough heat passes to effect the necessary mesomorphic transition of the ink. Thicker sponge layers are desirable in multi-use transfer elements since the sponge contains the ink; providing more sponge material provides a transfer element containing more ink, which results in a longer useful lifetime for the multi-use transfer element.
Ink compositions containing liquid crystalline materials are known. For example, U.S. Pat. Nos. 3,969,254 and 4,022,706, the disclosures of each of which are totally incorporated herein by reference, disclose cholesteric liquid crystal water base inks. Films formed from these inks can be used as temperature indicators and ornamental articles, and can also be used for photograph reproduction by irradiating through a negative placed on the film such that the exposed areas of the film undergo a temperature response change which is stable for long periods of time. Typically, the inks comprise an oil in water latex, a small amount of organic solvent, which is usually polar and moderately water soluble, liquid crystals, a thickening agent, and, optionally, a wetting agent.
In addition, U.S. Pat. No. 3,974,317, the disclosure of which is totally incorporated herein by reference, discloses thermometric compositions for recording changes in temperature which comprise a cholesteric liquid crystal system and a chemically inert substance immiscible with the crystal system. Varying the amount of the inert substance in the composition varies the temperature at which a predictable phase change to the isotropic phase occurs. All compositions in a particular product have the same color and exhibit the same color change.
Further, U.S. Pat. No. 3,666,947, the disclosure of which is totally incorporated herein by reference, discloses a liquid crystal imaging system employing an imaging member with a composition having a cholesteric liquid crystalline phase, with a radiation absorptive material dispersed throughout the liquid crystalline material. The imaging member is thermally imaged by heating portions of the material. The imaging members can also be erased by application of external forces, such as electric or magnetic fields.
Although the materials and processes in the prior art are suitable for their intended purposes, a need continues to exist for ink compositions suitable for use in thermal transfer printing processes. In addition, a need exists for thermal transfer printing elements that enable printing with reduced energy consumption. Further, a need exists for ink compositions suitable for use both in thermal transfer printing processes employing multi-use transfer elements and in thermal transfer printing processes employing single use transfer elements. There is also a need for transfer elements suitable for thermal transfer printing processes that require low amounts of applied heat to effect transfer to a substrate. In addition, there is a need for thermal transfer printing elements that enable rapid and inexpensive thermal transfer printing processes. Further, a need exists for thermal transfer printing elements that enable generation of high quality prints on plain paper.